In steam turbines, solid particle erosion damage to both stationary and rotating components in the steam path has become a very significant problem. The problem is exacerbated as the average in-service age of the steam turbines increases. It is known that a principal source of such erosion damage is the existence of iron oxide particles in the steam resultant from the exfoliation of oxides formed on the inner surfaces of the steam boiler tubes and steam piping at elevated temperatures and which particles impact on the nozzles and buckets along the steam paths. This solid particle erosion damage in steam turbines is a major contributor to problems associated with the operation and maintenance of steam turbines, for example, those used by utilities to generate electrical power. These problems include loss of sustained efficiency, forced outages, extended maintenance outages, cost of maintenance, cost of replacement parts and shortened inspection intervals. In fact, solid particle erosion damage has become such a contributing factor in the utilization of steam turbines for the generation of electrical power that a dollar cost per kilowatt hour per year is frequently assigned to this phenomena.
Efforts, of course, have been made to minimize or eliminate this problem. One approach has been to eliminate the source of the solid particles themselves, for example, by providing a chromium diffused layer on the internal surfaces of boiler tubes to inhibit formation of the oxides. While this solution may be effective in new steam turbines, it is not applicable for practical and cost considerations to units in-service. Other attempted solutions include acid cleaning of superheaters and reheaters to remove scale from the tube surfaces, and chromating boiler tubes. However, such methods to eliminate the problem at its source have proven expensive and oftentimes not practical.
Another approach to the solid particle erosion problem has been to produce steam path designs which are effective to resist such erosion. Recent studies have shown that the location and intensity of the particles impacting on the nozzles and buckets are the leading causes of erosion. For example, in the reheat section of a turbine steam path, the nozzles erode from the suction surface, particularly along their trailing edges as a result of particle collision with a leading edge of the bucket and rebound into the nozzle trailing edge suction surface. It is also known that the nozzle erosion caused by such particle rebounding phenomena may be significantly reduced or eliminated by increasing the axial clearance between the nozzles and buckets. This increased clearance affords more time for steam to accelerate the particles as they proceed from the nozzles to the buckets and for the steam to redirect the particles back toward the buckets after collision with the bucket leading edge. Thus, steam turbines have previously been designed with increased setback of the nozzles relative to the buckets. That is, the diaphragms of steam turbines where solid particle erosion is or is anticipated to be a problem, have been moved upstream relative to the buckets to increase the axial spacing therebetween and hence minimize or eliminate the problem.
With respect to double-flow reheat turbines, however, the provision of additional setback is replete with difficulties, particularly when modifying or retrofitting an existing reheat tub to provide such additional setback. Practical problems such as imperfections in original welds, the use of filler pieces to limit welding distortion in the original fabrication and a general inability to modify various components of the double-flow tub without causing other problems, for example, relocating external cooling pipes or upsetting rotor balance access ports, presents a formidable task if additional nozzle setback is desired in double-flow reheat tubs. In U.S. Pat. No. 5,249,918, of common assignee herewith, there is disclosed apparatus and methods for providing additional setback for the steam paths of a double-flow turbine. Essentially, material is added to and removed from the downstream and upstream sides of the diaphragms to enable the diaphragms to be displaced toward one another and further away from the first-stage turbine buckets. While this has proven satisfactory, the addition of weld material requires substantial fabrication, subsequent machining and hence high costs to produce a reliable double-flow steam turbine with additional setback.